Making a Microlens Array with Micromirrors

The best way to make a microlens may be to use a micromirror, according to researchers from the University of Texas at Austin. Associate professor Shaochen Chen and graduate student Yi Lu used a commercial digital micromirror device as a dynamic photomask to construct a microlens array. In a demonstration, they fabricated an array of spherical lenses, each nearly identical to the others.

The use of quickly changeable micromirrors could enable both desktop units and larger setups that are suitable for volume manufacturing. The technique could overcome a problem typically encountered when making microscale optics: Their height can be constrained by the photoresist thickness, and that limits their numeric aperture. “This technique could provide a solution to making high-aspect-ratio optics,” Chen said.

A computer-controlled digital micromirror chip modulates incoming light as it is reflected into a vat of liquid photosensitive polymer. As a result of the exposure, microlenses form on the substrate. Images reprinted with permission of Applied Physics Letters.The new approach arose from an earlier study in which the researchers exposed a photosensitive resin on a glass substrate to a series of illumination patterns at various intensities. Some of the resulting structures looked like the microlenses that boost the performance of organic LEDs, photovoltaic solar cells and CMOS imaging devices.

Many current microlens manufacturing techniques that do not depend on a photomask can produce only nearly spherical microlenses of specific sizes, a restriction that can get in the way of optical performance. Direct writing or gray-scale masks also can be used to make microlenses. The former is slow, while the latter is limited by the height of the structures that can be built.

In their technique, the researchers illuminate a computer-driven digital micromirror device. The reflected light travels through a lens and a transparent substrate, which covers — and is in contact with — photocurable resin in a vat. The resin solidifies in response to the light. The formation of the resin on the underside of the substrate is important, Chen noted. “It is critical to expose through a transparent substrate in order to achieve a high aspect ratio.”

A scanning electron microscopy image reveals the high degree of uniformity in the microlens array that was formed with the help of a digital micromirror. The inset shows the profile of an individual lens.In a demonstration of the technique, the researchers used a commercial projector from BenQ Corp. of Taipei, Taiwan, that is based on a Texas Instruments digital micromirror device. They used an ultraviolet spot-curing system from American Ultraviolet Co. Inc. of Lebanon, Ind., as a light source and made the photoresin by mixing a photosensitive component in a polymer.

They exposed the photoresin to UV light — moderated by the digital micromirror device — with a typical exposure time of a few seconds. After the exposure, they took the substrate out and removed uncured resin. To ensure uniform photopolymerization, they flooded the array with UV radiation.

Microlenses produced with a 370-mW/cm2 intensity at the center measured 230 μm in diameter, and the array had a pitch of 510 μm. The focal length — as determined by imaging a 633-nm helium-neon laser made by Coherent Inc. onto a JAI model CV-S3200 CCD camera — was 300 μm. The average surface roughness was about 3 nm, as measured by a Dimension 3100 atomic force microscope from Digital Instruments.

The product actually was smoother than it should have been, which the researchers attributed to a thin layer of uncured resin present at the time of the UV flood exposure.

The investigators now are developing models to improve the accuracy with which they can predict the parameters that are needed to create particular structures in the photoresin. They also are developing digital micromirror exposure systems. The scientists have completed a second-generation device, with one of their goals being the creation of a more versatile imaging system.

“We are looking forward to combining digital mirror device-based techniques into a technology platform, which will include various formats of stereolithography and gray-scale lithography,” Chen said.